The invention relates to a method for fabricating a semiconductor substrate starting from a silicon on insulator (SOI) type substrate.
In the optoelectronics, special substrates are needed which are for example used for image sensors, which find their application in video or photographic cameras. In these substrates, which are based on SOI type substrates, a buried oxide (BOX) is provided on a base through which photons can be collected from the backside of the wafer. Alternatively, the image sensors formed in the device layer of the SOI substrate could be transferred to a final substrate to expose the backside of the sensors. A thin highly doped p++ (or n++) first semiconductor layer is directly provided on the buried oxide and a second semiconductor layer with a lower dopant concentration (p−/n− layer) is provided on the first semiconductor layer.
In the prior art, this kind of special substrate was prepared using a highly doped substrate as the donor wafer in the conventional Smartcut™ technology. This method typically comprises the steps of providing a donor substrate, e.g. a silicon wafer, providing an insulating layer on the donor substrate and creating a predetermined splitting area inside the donor substrate, which is achieved by implanting atomic species or ions, like helium or hydrogen ions, into the donor substrate. In the next step the donor substrate is bonded to a base substrate, e.g. a further silicon wafer, such the insulating layer is sandwiched between handle and donor substrate. Subsequently, the remainder of the donor substrate is detached from the bonded donor-base substrate at the predetermined splitting area following a thermal and/or mechanical treatment upon the predetermined splitting area. As a result a semiconductor on insulator (SOI) substrate is obtained.
However, the use of a highly doped substrate can lead to the following problem: It can be observed that cross-contamination from one donor wafer, e.g. with a high dopant concentration, to another wafer, e.g. with a lower dopant concentration like in the standard SOI substrates, occurs within the production line. This leads to unsatisfying dopant profiles in both the p++ type SOI wafers and the standard p− SOI wafers. In addition, during subsequent annealing steps during the SmartCut™ process, a diffusion of dopants out of the highly doped layer occurs which further deteriorates the substrate.
According to an alternative method a standard SOI substrate with a thin p− (or n−) semiconductor layer was used as starting material over which a further semiconductor layer with a p++ dopant concentration was provided. Finally, an additional semiconductor layer with p− concentration was provided to obtain the desired layer structure. With this method it is, however, not possible to reach a sufficient high dopant concentration in the p++ layer and furthermore the dopant concentration is not sufficiently flat, meaning that that the dopant concentration through the layer is not monotonic, but first increasing and then decreasing within the p++ layer.